1. Field of the Invention
This invention relates to a belt type continuous speed-shifting device to be used for agricultural implements and other machines.
2. Description of the Prior Art
Conventionally, a belt type continuous speed-shifting device is composed of a variable speed pulley mechanism having variable speed pulleys supported on each of a pair of rotary shafts arranged in parallel with each other. Each of the above pulleys consists of a fixed sheave fixedly rotatably and axially non-slidably mounted on the rotary shaft and a movable sheave fixedly rotatably and axially slidably mounted on the rotary shaft, facing the fixed sheave. A V-shaped belt groove is formed between the fixed sheave and the movable sheave and a belt is trained between belt grooves of both variable speed pulleys. The speed change ratio between both rotary shafts is changed by varying an effective radius in relation to the V belt by moving each movable sheave in axial direction.
For example, the Japanese Patent Application Laying Open Gazette No. 62-118159 discloses a speed-shifting device which is provided with the above-mentioned variable speed pulley mechanism and a planetary gear mechanism (differential gear mechanism) as a gear mechanism for speed-shifting.
The above speed-shifting device with a variable speed pulley mechanism and a differential gear mechanism is high both in theoretical transmitting efficiency and actual transmitting efficiency. However, a problem with this device has such trouble that if an output shaft is made to rotate both in normal and reverse directions, driving power is applied to the belt of a pulley mechanism in one of the rotating directions and therefore too large force is applied to the belt at the time of high output, though it matters little at the time of low output. More particularly, in the closed-circuit type differential gearing, one of three gear elements of the differential gear mechanism is connected to an output shaft and by varying the number of revolutions of one of the remaining gear elements by regulating the speed change ratio at the pulley mechanism, the rotating direction and the speed of rotation are differentiated between said gear element and the remaining other gear elements and thus the rotating direction and the number of revolutions of the gear element on the output side, namely, the output shaft, are determined. At this time, however, both driving power and circulating power are generated as motive power and output power is equal to the driving power minus the circulating power. Which of the two power transmitting channels will become a driving power channel or a circulating power channel depends upon the angular speed of gear elements in the differential gear mechanism and the gear element of larger angular speed becomes the driving power channel. Since driving power is larger than circulating power, if it is tried to transmit small circulating power on the pulley side at the normal rotation, for example, at the reverse rotation the speed change ratio of the pulley is reversed and therefore large driving power is transmitted to the pulley side. This involves the problem of durability of the belt at the time of high output.
In order to solve the above problem, it is suggested to add a clutch and a normal and reverse rotation gear and by switching the normal and reverse rotation gear, small circulating power is always applied to the variable speed pulley mechanism. However, use of the normal and reverse rotation gear will require more space and will involve a cost increase.
The present invention has been made in view of the above and has for its object to provide a continuous speed-shifting device of high transmitting efficiency, low cost and space saving by applying small circulating power to the belt of the variable speed pulley mechanism only by a clutch, without using a normal and reverse rotation gear.